Field of the Invention
The invention relates to an apparatus and a method for producing a chip-substrate connection, in particular by soldering a semiconductor chip on a substrate, which apparatus has a support, on which the substrate is temporarily supported, and a heating device assigned to the chip-substrate connection.
When a rear side of a semiconductor chip is connected to a substrate, which is usually referred to as chip or die bonding, the requirements with regard to sufficient mechanical fixing and also good thermal and electrical conductivity must be fulfilled individually or jointly, depending on the application. An important factor is the compatibility of the chip and the substrate, that is to say the matching of the two participants in the connection in terms of their expansion behavior under thermal loading. At the present time, essentially three distinguishable methods of chip fixing are customary: alloying (hard soldering), soldering (soft soldering), and bonding. The preferred area of application according to the present invention is soldering (soft soldering), in the case of which method principle, in contrast for instance to eutectic Si/Au hard soldering for the chip fixing, no silicon of the chip goes into the solution. Rather, in the course of soft soldering at temperatures distinctly below 450xc2x0 Celsius and thus also again significantly below the melting point of the metals involved, the metallized participants in the connection are wetted by melting solder and then connected. Although the temperatures that occur in the course of soft soldering allow the method to be applied to all customary substrates, it is preferably applied to metallic lead frames and ceramic housing bases. In general, in the case of chip fixing by soldering, matching in terms of the thermal expansion behavior of the participants in the connection is desired, but the limits are set more widely than in the case of alloying.
An untreated rear side of the chip cannot normally be wetted by soft solder, this must be metallized. In addition to the requirement, applicable in this case as well, for good adhesion on the silicon and a junction free of a depletion layer, there is also the requirement for good solderability, that is to say wettability. Thus, for junctions which are free of a depletion layer, principally solders containing Ti, NiSi and AuSb have proved worthwhile on n-Si layers and solders containing Al, Cr and Au on p-Si layers. The substrate surface is usually nickel-plated by chemical methods. If low soldering temperatures are desired, silver electroplating can also be considered.
Resistance-heated soldering receptacles or heating windings have been used hereto for as heating sources for heating the semiconductor chips and materials to be connected. However, the heating sources have the disadvantage that their rate of heating up to the soldering temperature is too long (generally significantly longer than 1 s), with the result that oxidation of the solder takes place and, consequently, it is necessary to use protective gases or reducing atmospheres.
Published, European Patent Application EP 321 142 A2 discloses a diode laser-assisted soldering method in which a heating element which engages around the parts to be connected in a bell-shaped manner and is made of a material that absorbs laser radiation is provided. The heating element is heated by irradiation with laser light and outputs thermal energy to the parts to be connected. The heating element is not in direct mechanical contact with the parts to be connected, rather openings are provided which ensure sufficient circulation of air.
Published, British Patent Application GB 2,244,374 A discloses a method for the laser-assisted soldered connection of a semiconductor wafer substrate and of an integrated circuit in which the infrared radiation of the laser is directed through the wafer substrate directly at the connection to be soldered.
Further laser-assisted or radiation-assisted soldering methods have been disclosed in IBM TBD vol. 22 No. Jul. 2, 1979 and IBM TDB vol. 24 No. Jan. 8, 1982.
It is accordingly an object of the invention to provide an apparatus and a method for producing a chip-substrate connection which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for producing a connection, including:
a heating device for forming a chip-substrate connection, containing:
a support for temporarily supporting a substrate with a semiconductor chip disposed thereon, the support having a heat body that can be heated by electromagnetic radiation, the heat body being thermally intimately coupled to the substrate; and
a radiation source supplying the electromagnetic radiation to the heat body for connecting the semiconductor chip to the substrate.
The invention is based on the object of specifying an apparatus and a method for producing a chip-substrate connection, in particular by soft soldering for fixing a semiconductor chip on a substrate such as, for example, a metallic lead frame or ceramic housing base, in which heating up for the purpose of producing the soldered connection to more than, typically, 350xc2x0 Celsius is enabled in a very short time, that is to say approximately 1 s or less, and, at the same time, of keeping the thermal loading on the affected semiconductor chip as low as possible.
The invention provides for the heating device to have a radiation source, and for the support for supporting the substrate to have, as part of the heating device, a heat body, which is assigned to the chip-substrate connection and is heated with electromagnetic radiation by the radiation source during the operation of the apparatus. Following the principle of the invention, it is provided that the radiation source constitutes a laser outputting laser radiation in the infrared wavelength range.
By comparison with the heating sources used heretofore in the case of soft soldering for chip fixing, the invention affords, in particular, the below recited advantages.
The use of laser radiation in the infrared wavelength range typically around 950 nm results in that the soldered connection of the semiconductor chip and the substrate can be heated up to more than approximately 350xc2x0 Celsius in a very short time, that is to say typically in approximately 1 s or less. The laser power used being chosen appropriately in accordance with the heat body heated with the thermal radiation, in such a way that the heat body exhibits sufficiently high absorption of the laser radiation in order that the infrared light energy can be converted into thermal energy, but on the other hand the laser power is not so high that the semiconductor chip to be connected is exposed to excessively great thermal loading.
Oxidation of the solder is avoided as a result of the very high rates of heating up to the soldering temperature, with the result that it is possible to work even without the use of protective gases or reducing atmospheres. The use of soldering fluxes can be obviated on account of the very short time intervals until the soldering temperatures are reached.
The brief heating can be effected in an approximately spot-type manner and thus in a chip-by-chip manner, with the result that only the respectively affected semiconductor chip for producing the soldered joint is heated in a controlled manner, and the semiconductor chips adjacent to it do not yet experience any significant temperature increase.
The term xe2x80x9cchip-substrate connectionxe2x80x9d should be understood to be not only connections of individual chips cut or sawn from the wafer composite but also connections of a plurality of still contiguous semiconductor chips from the wafer composite, so-called chip ingots.
It is advantageously provided that the heat body, assigned to the chip-substrate connection, of the heating device has a material that is transparent or at least translucent to the thermal radiation from the radiation source. Quartz as the preferred material for the heat body has the advantage, on account of its low degree of expansion, that short, locally occurring temperature fluctuations do not lead to cracks or breaks.
Following the principle of the invention, it is furthermore provided that on its side facing the chip-substrate connection the heat body has a material exhibiting high absorption of the light radiation fed by the radiation source. The material exhibiting high absorption is particularly advantageously provided as a thin metal layer coating, in particular a thin chromium layer, on that surface of the heat body that faces the chip-substrate connection. In the case of a transparent heat body such as quartz, for example, the thickness of the applied chromium layer is advantageously a few 100 nm, at most up to approximately a few xcexcm, or more, provided that there are no problems to be expected with regard to sufficient adhesion of the chromium layer on the heat body. The material exhibiting high laser absorption on the heat body affords, according to the invention, above all the advantage of achieving sufficiently high absorption of the laser radiation in order to convert the infrared light energy into thermal energy. At the same time, the good absorption of this material results in that it is possible to reduce the laser power used, thereby opening up, in turn, the use of the already available, particularly well suited laser sources such as, in particular, semiconductor laser diodes for producing a chip-substrate connection according to the invention.
In the case of a heat body coated with the laser absorption material, the heat body itself should be a poor heat conductor and ideally transparent to the laser radiation, or, in the case of good thermal conductivity, should have a sufficiently low thermal capacity. In this way, the proportion of energy not used for heating up the primary materials can be kept low in both cases. In this sense, too, preference is given to a heat body made of quartz that is provided with a chromium layer serving as an energy absorber.
Particularly advantageously, it may be provided that the extent of the material exhibiting high absorption is limited to that section of the directly affected semiconductor chip of the chip-substrate connection that is to be heated. This opens up the possibility of bonding the chips individually without unnecessarily heating up adjacent chips.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus and a method for producing a chip-substrate connection, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.